The present invention relates to single phase brushless DC motors, i.e. a motor with a single coil. Single-phase motors are typically used in low cost motor applications, such as fan cooling applications, where typically the speed control loop is closed by a remote CPU, while the motor driver itself basically converts an input PWM signal into an output PWM signal according to some look-up table or transfer function.
For many applications, three phase brushless DC motors are preferred because of their lower torque ripple, leading to lower noise, higher efficiencies and higher start up torque. But single coil motors are cheaper to produce and to drive, and therefore preferred in some high-volume markets, such as e.g. fans for cooling CPU's in desktops, refrigerators, printers, as a few examples only.
Brushless DC motors have the advantage that no brushes are needed, but they require a specific driving scheme, called “electrical commutation” to change the direction of the current through the coil(s), which is a principle well known in the art.
Although there are communalities between single phase brushless DC motors, dual phase brushless DC motors and three-phase brushless DC motors, there are also important differences. A first difference is that the torque of a single-phase or two-phase motor varies quite differently from that of a three phase motor. FIG. 1(a) shows a typical torque as a function of rotational position for a single/dual coil motor, in case of permanent energization of the motor. It is well known in the art that dedicated energization schemes may slightly influence the torque curve to improve torque ripple and noise. These schemes are often referred to as “soft switching” for single/dual coil and “trapezoidal or sine wave control” for three phase BLDC. Dedicated sinusoidal 3 phase motors driven with dedicated sinusoidal energization schemes may even result in virtually zero torque ripple. FIG. 1(b) shows a typical torque as a function of rotational position for a three-phase motor in case of permanent energization.
The fundamental reason for the difference in torque between both motor types is that in a three-phase motor the permanent energization over a full electrical rotation is divided in six regions of 60° each, whereby at least two coils are energized simultaneously in each of the six regions. The current in the coils interacts with the permanent magnetized rotor, leading to the torque curves such as shown in FIG. 1(b). In contrast, in a single coil motor, there is only one coil. During the electrical commutation of this coil the total motor current is decreased to zero before ramping it up again in the opposite direction, which explains why the torque curve of single/dual phase motors always has to pass the zero torque point shown in FIG. 1(a). Brushless Motor control, both for single coil as well as for 3-phase motors, implies the capability to align the electro magnet with the rotor magnet, in other words to align the commutation of the motor current in the stator coil to the position of the rotor magnet. In practice the transition of rotor magnetization from north to south pole requires a certain number of electrical degrees. Because there is no additional coils to provide torque during such transition for single coil motors the possible torque reduces, down to zero at the exact zero crossing. The coil current is defined by the applied supply voltage VDD, minus the back EMF voltage (bemf) induced into the stator coil by the moving rotor magnet, and by the motor parameters Lcoil, Rcoil, and the motor driver resistance. When the magnetic field drops in the transition phase (around the BEMF zero crossing) this will lead to a rise in the coil current. This current cannot interact with magnetic fields, and therefore leads to reduced efficiency. Additionally the current needs time to change direction in the coil. The increased coil current around a BEMF zero crossing will further add to the time required to change the direction of the current, and leads to so called reverse current after commutation. This reverse current causes supply ripple, and generates a braking effect in the fan. The consequent high torque variation causes vibrations and acoustic noise. Soft switching refers to a current control method which aligns the coil current commutation with the rotor position to minimize the above effects.
Another difference is that three phase BLDC motor controllers with sensors require three hall elements located at exactly 60 or 120 electrical degrees to discriminate between the six possible motor states, while single/dual coil motors only require one single hall element to discriminate between the two possible motor states, requiring only limited production mounting tolerances. This allows even to integrate this hall element into the motor driver, leading to further miniaturization and cost reduction.
A specific market requirement of speed control fans is the definition of the speed curve. This speed curve defines how an input signal, for instance a duty cycle input signal (DCin) is converted into a resulting fan motor speed. In case of fan-drivers which control power stages using PWM to control the coil current, the coil energization is the result of a waveform generated. In basic implementations the waveform can consist of a single output duty cycle (DCout). In more complex implementations the waveform can consist of varying output duty cycles in order to realize so called soft switching. For simplicity to refer to both cases, basic and complex, we refer to a single theoretical output duty cycle DCout as a reference of the energization level targeted by the applied waveform. In practice DCout can also be the maximum level of the soft-switching waveform.
Because such low cost fan-drivers don't regulate speed, but only apply a requested output duty cycle (DCout), the speed resulting from the energization depends on the fan design, such as the blade design, and system environment, such as back pressure. The lack of closed loop speed control, and the non-linear increase of the load as a function of the speed, causes that the speed curve is a non-linear function of the output duty cycle (DCout). Several applications accept a natural speed curve in which the percentage value of Dcin is equal to that of DCout. This relationship is quite easy to realize in a state-machine and results in very low-cost speed controlled fan-drivers. Some applications however such as CPU or GPU cooling have more complex requirements.
The latter applications define a start point P1 and an end point P2 of the target speed curve. A linear speed change between those points is requested (desired). And deviations may vary within a given boundary, for instance from +/−200 rpm at P1, to +/−10% at P2. The speed curve starting point P1=(DCin_0, DCout_0) may be requested in a wide range, for instance (0%, 40%) as well as (40%, 10%). In other words, according to the specification, the actual speed curve needs to be located between an upper boundary 42 and a lower boundary 41 (see FIG. 4), which is difficult because the speed curves are typically quite non-linear. Some examples of curves which do, or do not fall between the boundary lines, are shown in FIG. 4. In order to limit inventory of different fan-drivers for different end customer requirements, the speed curve DCout=F(DCin) of state of the art fan-drivers for CPU/GPU cooling fans can be tuned using external components in order to match the requested target speed curve. This leads to larger pin-count packages such as TSSOP16. Attempts to reduce the pin count to SOIC8 imply also reduced tuning capability. An example of such a target speed curve is shown in FIG. 3.
FIG. 2 shows a typical system configuration where a remote processor 21 provides a duty cycle signal (DCin) as a PWM-signal, indicative for the desired fan speed, and whereby the single coil motor driver has to drive a fan motor such that the actual speed of the fan is substantially proportional to the duty cycle value (relative to its maximum speed).
While it would be theoretically possible to implement a closed-loop system in the fan driver, e.g. by embedding a controller with a look-up table to implement a non-linear transfer function that compensates for the non-linear load, such sophisticated solutions are not viable for cost reasons. And while it would be possible to implement the closed loop in the CPU shown in FIG. 2, this is not what the market asks.
It is a challenge to provide a single coil motor driver of low complexity that, when used in combination with a fan motor, provides speed curves located between the specified boundary lines as shown in FIG. 4.
FIG. 5 illustrates how the problem is addressed today, namely by providing fan drivers having multiple linear transfer functions in a duty cycle convertor block converting the “duty cycle input signal” (DCin) into a so called “duty cycle output signal” (DCout), (which actually is an internal signal) and one of these transfer functions is selected by means of external components (e.g. resistors and/or capacitors). In fact, the three fan speed curves shown in FIG. 4 correspond to the three transfer curves shown in FIG. 5 used in conjunction with a particular fan. As can be seen in FIG. 4, only one of the three curves falls between the boundary lines, hence that transfer function would have to be selected for that particular fan to satisfy the requirements.
However, there is room for improvement and/or alternatives.